Feeding Horses for the Summer

with Equine Nutritionist David Nash

In Australia and in the tropics of Asia horses endure severe extremes in temperature and humidity. This can dramatically affect horses’ health and performance if not managed correctly.

High temperature, high humidity, lack of air movement, poor ventilation, dehydration and exposure to direct sunlight all increase the danger of serious heat and sun related problems for humans and horses are no exception, especially when they are expected to perform at intense levels.

Signs of Heat Stress

During exercise, there is a significant increase in the amount of heat produced by working muscles. Muscles cannot transform energy into movement with 100% efficiency. Horses transform energy to movement at approximately 25% efficiency. As a result, some of the energy is lost in the form of heat. The rate of heat production by working muscles is proportional to how hard the muscles work. Therefore the faster a horse moves the more heat it produces. The amount of heat a horse produces in a 160 km endurance race would be enough to boil approximately 770 litres of water. That’s approximately 4.8 litres per km. Fortunately for the horse, it is able to dissipate around 97% of the heat it produces during exercise in cool-warm conditions. If not, its body temperature would increase by around 15°C/h. In response, a horse increases its sweating rate, moves more blood to the capillaries under the skin and increases its rate of breathing in an effort to release this build up of heat.

Commonly observed signs of heat stress are:

  • Profuse sweating
  • No sweating
  • Rapid breathing rate – panting (>20 breaths / min)
  • Rapid heart rate (>50 beat/min)
  • Skin that is dry and hot
  • Unusually high rectal temperatures (>38°C)

A simple pinch test can basically determine whether a horse is dehydrated as a result of heat stress. When you pinch the horse’s skin on the neck, it should resume its original position immediately. If the skin takes a while to resume its normal position it could be assumed that the horse is somewhat dehydrated.

Nutrition and its Role in Reducing Effect of Heat

Nutrition can play an important role in reducing the thermal load of horses enduring heat stress. Many equestrians would know about “cool feeds”, but what we are referring to is feeds that do not create a lot heat as a result of fermentation in the hindgut. This is done by processing the grains with heat. Practices such as Micronizing enable a majority of the starch in the grain to be digested in the small intestine, which greatly reduces heat production via fermentation in the hind gut.

Micronizing improves feed efficiency, thus less feed is required to be fed to the horse. Also the Micronizing process actually enhances the palatability of grains and is a useful tool to help keep horses eating during times of environmental stress.

The type of roughages used in a horses’ diet also determines the amount of heat which will be produced via fermentation. Soft leafy roughages such as quality grass hay will produce less waste heat than stalky oaten hay as the fermentation process has to work a lot harder to digest stalky roughages.

Fat is digested quite efficiently in the horses’ small intestine and produces very little heat whilst being digested. The problem however is that high fat feeds and oils can quickly go rancid if not stored correctly in a cool dark environment. As fat is high in calories it can reduce the amount of total feed required to meet the horses requirements per day, reducing the amount of fermentation occurring. Recent studies into the addition of fat into a grain diet resulted in the slowing of feed travelling through the digestive tract. This allows enzymes in the small intestine more time to digest both the fat portion of the diet but also the starch component resulting in less starch entering the hindgut for fermentation.


A racehorse can lose up to 10 litres of sweat per performance (work/race). This fluid isn’t just water – it contains a lot of salt. These salts, when broken down into their chemical components, are referred to as electrolytes. These are typically groups of different salts that contain such electrolytes as sodium, potassium, chloride, magnesium and calcium. Electrolytes govern the transfer of water through cell membranes into or out of the cells. Thus, they function in getting the nutrients in and the waste products out. They are responsible for getting nerves to fire and muscles to contract. Essentially all of the physiological actions in the body require electrolytes. And importantly, they need to be present in the fluids in the appropriate amounts for these biochemical reactions to proceed in an orderly manner.

If we don’t provide at least a minimum electrolyte replacement, horses can suffer such medical conditions as metabolic alkalosis, inefficient transport of oxygen and energy substrates, poor tissue perfusion, thumps, muscle spasms, exertional rhabdomyolysis (tying up), cardiac arrhythmias, gastrointestinal stasis, anhidrosis, kidney impairment, and poor recoveries. (Actually, poor heart and respiratory recovery is one of the key signs that can lead to the recognition of the problems associated accomplishing thermoregulation.) The point is, most of these problems mentioned stem from the resulting dehydration and electrolyte imbalance.

The real question becomes, how much better could the horse perform if it were in a state of ideal electrolyte and fluid balance? How many horses that fade in the last third or an event, or have prolonged recoveries after an event, could be winners if their electrolytes and fluids were balanced and at appropriate levels?

Thus, it is essential to correctly manage and supplement horses’ diets with fluids and electrolytes. In the commercial world of equine nutrition there is a plethora of electrolytes available. It is wise to carefully examine the labels of these products as many contain vast amounts of fillers and incorrect ratios of electrolytes.

For the full article on heat stress in horses or further nutritional advice please contact David Nash on or

Thanks to David Nash
Hygain Feeds